The invention relates generally to data communications and more particularly to high speed data transmissions over the public switched telephone network.
The sudden popularity of the Internet as a communication tool has led to an intense push for higher data transmission rates over the Public Switched Telephone Network (PSTN). As a result, the demand for increased data transmission rates over analog twisted pair wiring is at an all time high. The most recent widespread standard is xe2x80x9c56Kxe2x80x9d analog modem technology developed by U.S. Robotics and Rockwell/Lucent. While these technologies will not generate true 56 kbps performance under typical subscriber line conditions, they do provide a boost in performance from the previous standard of bidirectional 33.6 kbps.
Theoretically, a connection of 64 kbps should be attainable between the subscriber and the Internet Service Provider (ISP) via a standard Plain Old Telephone Service (POTS) connection. This is because 64 kbps is the rate at which data is transferred from the Central Office (CO) linecard to the ISP or other remote terminal. Several factors prevent this from happening including imperfect line conditions and varying local loop lengths common to POTS analog networks. The primary reason, however, for this less than the theoretical transmission rate is that the PSTN was designed to carry voiceband frequencies in the range of 300-3.4 KHZ.
With the advent of digital voice systems, the decision was made to use a xe2x80x9ccompandedxe2x80x9d (compressed/expanded) data to reduce the number of bits per digital sample from a nominal 13-bits to 8-bits. Companding schemes use higher resolution at low signal amplitudes and lower resolution at high amplitudes. Companded signals are suitable for voice frequencies but not for analog modems since they limit their apparent bandwidth to a ceiling of 33.6 kbps upstream and 56 kbps downstream. In practice, most analog modems are only able to achieve rates of 46-48 kbps downstream due to less-than-perfect analog line conditions.
The Analog-to-Digital (xe2x80x9cA/Dxe2x80x9d) portion of the linecard codec is where the analog signal is converted to its 8-bit companded representation. Hence, the linecard codec acts as a bottleneck in the entire data communications chain. One way of avoiding this bottleneck is by removing the A/D conversion in the downstream direction. This is accomplished by requiring a digital connection between the provider and its CO and increasing the data throughput of the modem signals to capitalize on the extra capacity.
Moreover, while the use of 56K standards results in downstream data throughput of 56 kbps under ideal local loop conditions, the upstream direction must still contend with an A/D conversion into 8-bit companded data and is still limited to 33.6 kbps. Imperfect conditions in the analog local loop further degradate the signal resulting in less than the 56/33.6 kbps maximums.
Additionally, while 56K standards offer improvements over the older V.34+ standard, bandwidth is still needed to keep pace with upcoming technologies such as video conferencing, remote server access, and others high rate transmission protocols. If higher data throughput is to be achieved, the limitations in the CO need to be overcome. Overhauling the PSTN by replacing the 8-bit companded data scheme could solve the problem, but this is not a feasible solution since the cost of such as effort would be enormous.
The invention overcomes a limitation in bandwidth of prior communications standards including 56K by offering increased downstream rates using an analog modem communicating over the PSTN.
Disclosed in one embodiment, is a method of requesting bandwidth on a digital communications channel. The method comprises the steps of determining the present data communications activity occurring over the digital communications channel and then calculating the bandwidth requirements of the channel based on the present data communications activity. Next, a predefined code pattern is formatted containing a bandwidth request and transmitted to the central office. The central office linecard codec decodes the code pattern to reveal the bandwidth request and, in turn, transmits the request to a third party administrator of the digital communications channel.
According to another embodiment, a method of requesting multiple timeslots for communications over a digital backplane is further disclosed. The method is used in combination with a central office linecard that permits communications from a modem coupled to the Public Switched Telephone Network (PSTN) to a service provider. The method comprises the steps of establishing a voice call between the modem and the central office linecard and negotiating a communications link between the service provider and the subscriber modem. A command code is transmitted to the linecard and the linecard is disconnected from the communications link with the subscriber modem. Next, a code pattern containing a request for timeslots is transmitted to the linecard which, in turn, is transmitted to a third party administrator of the digital backplane.
Also disclosed is a method of de-allocating unused timeslots. In one embodiment, when the service provider has determined that a multiple-timeslot connection has been idle for a period of time, it transmits a code to the subscriber modem requesting a move to a lower rate protocol such as V.34. Upon successful transition, a predefined code pattern is transmitted to the CO linecard codec from the service provider. The linecard codec, recognizing the code pattern, initiates a move back to xe2x80x9cvoicexe2x80x9d mode, releasing the extra timeslots allocated to it. The linecard codec can then move back into xe2x80x9cdataxe2x80x9d mode when more timeslots are required.
Further disclosed is an improved linecard codec supporting multiple timeslots on a digital backplane. The linecard permits increased data rates between a subscriber and a service provider over the PSTN. In one embodiment, the linecard comprises an analog interface coupled to the PSTN, a digital interface coupled to a digital backplane used by at least one service provider, a conversion circuit interspersed between the analog interface and the digital interface, and a controller circuit coupled to the digital interface and configured to request multiple timeslots from a third party administrator of the backplane.
According to another embodiment, the linecard codec incorporates an input register that receives requests for timeslot allocation from the service provider. Code patterns from the service provider are decoded by the linecard codec which, in turn, formats a request for bandwidth. The linecard codec allows the backplane to temporarily revert to xe2x80x9cvoicexe2x80x9d mode during periods of inactivity. This causes more efficient use of the communication network. Since a large portion of the time spent during common Internet access is inactive, the total data rate is increased.
According to another embodiment, the linecard codec contains a code recognition mechanism that provides a way to dynamically allocate and deallocate timeslots during data communications. A certain amount of intelligence is employed in the code recognition mechanism to handle simple handshaking and act on the code pattern received from the service provider. The general instruction architecture places the service provider in the master position and the linecard codec in the slave position.
According to another embodiment, the linecard codec incorporates the control logic responsible for formatting requests for more or less timeslots and transmitting then to the network administrator. During periods of inactivity, the timeslots are deallocated to make room for other connections on the same backplane.
According to another embodiment, the linecard codec includes a strobe terminal that permits sending interrupts to the linecard microcontroller for xe2x80x9con-the-flyxe2x80x9d timeslot allocation.
A technical advantage of the invention is that it provides the subscriber with much more bandwidth than currently available with analog modems while maintaining much of the same equipment and connection methods.
Another technical advantage of the invention is that it permits replacement of the existing linecard in the CO with the linecard of the present invention enabling hardware and software changes at the CO to provide the increased bandwidth.